This application claims the priority German patent document 199 24 947.4, filed May 31, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and an apparatus for limiting the traveling speed of a motor vehicle.
Cruise control systems are known in which a maximum speed preset by the driver or calculated via an automatic determination system can be maintained automatically in a motor vehicle. Such cruise control systems include both Tempomat functions, in which the vehicle is intended to maintain a minimum speed selected by the driver, and limiter functions, in which the vehicle must not exceed a maximum speed that is preset by the driver or is determined automatically (for example, by a proximity control system). In cruise control systems, control signals are generated based on the maximum speed and the actual vehicle state variables (in particular, the actual vehicle speed), and are used to adjust the engine, the transmission or the wheel brakes in order to maintain the maximum speed.
In German patent document DE 44 07 082 A1 a motor vehicle automatic transmission is controlled to brake the vehicle from its instantaneous speed to a desired maximum speed which is determined, for example, based on the distance from a vehicle traveling in front. For this purpose, a change to a lower gear is forced to occur, thereby achieving a higher transmission ratio, with a greater deceleration level. In this system, the driver can operate the accelerator pedal to deactivate the automatic proximity or cruise control and to vary the distance from the vehicle traveling in front as he himself desires.
A cruise control system with a limiter function is disclosed in German patent document DE 195 09 492 C2, in which the driver can preset a maximum permissible speed. The control system automatically limits the vehicle speed to the maximum speed when operation of the accelerator pedal by the driver would result in a vehicle speed greater than the selected maximum speed. In this situation, a control signal which corresponds to the maximum signal and is used to vary the engine and the transmission is produced irrespective of the actual accelerator pedal position.
If the preset maximum speed is manually deactivated by the driver, the engine torque is increased from the last value of the control signal produced by the limiter system to a value which corresponds to the actual accelerator pedal position. If the actual accelerator pedal position is significantly greater than the last value of the control signal from the limiter system, then the system attempts to increase the engine torque in as short a time as possible to such an extent that the actual and desired engine torque match. This effort to produce a high engine torque causes the automatic transmission to shift down to lower gears while the engine speed at the same time increases. This can be annoying to the driver, who in some circumstances is not aware of the increased accelerator pedal position, and can possibly cause incorrect reactions and dangerous situations.
German patent document DE 44 34 022 A1 relates essentially to processes which take place while speed limitation is activated. In the kick-down mode, the transmission is controlled so that gear selection is limited to second gear, it being impossible to change down to first gear. However, such downshifting is feasible only in the lowest speed range, for example if the speed limiting is preset to 30 km/h. If, on the other hand, the speed limitation in the control system is set to a higher speed, first gear cannot be selected anyway.
This document does not refer to any more far-reaching transmission actions, in particular for the situation where the actual accelerator pedal position at the time when speed limitation is switched off is greater than the accelerator pedal substitute variable.
The object of the present invention is to improve driving safety in vehicles that are equipped with an automatic transmission and a speed limiter system.
This and other objects and advantages are achieved by the method and apparatus according to the invention, in which a low gear limit is introduced which takes effect at the time when the cruise control system is switched off, in order to prevent the automatic transmission from automatically changing gears after deactivation of the speed limiting system in a manner which is objectively and subjectively unpleasant or is dangerous. Those situations in which undesired gear changes are to be suppressed, or at least limited, are identified automatically by the system. These are situations in which, at the time when the system is switched off, the actual accelerator pedal position input by the driver is greater than the value of an accelerator pedal substitute variable. The latter variable is preset by the limiter system, based on the selected maximum speed and, when the control system is activated, is used by the limiter system as the basis for setting the transmission, and possibly the engine and the brakes. The comparison between the actual accelerator pedal position and the accelerator pedal substitute variable provides a reliable basis for the decision whether to implement a low gear limit after the system is switched off. The accelerator pedal positions correlate with engine torque; accordingly, a direct comparison of the actual engine torque applied by the speed limiter system and the engine torque corresponding to the actual accelerator pedal position can also be used as the basis for the method and the apparatus for controlling the automatic transmission.
The gear limit is produced by presetting a defined minimum gear, whose level depends on the vehicle state and operating variables (in particular on the actual vehicle speed), the load, the road gradient or the road condition etc. Addition variables which influence the gear limit may take account of electronic driving actions which stabilize the vehicle, such as drive slip control, an antilock braking system or the like.
According to the gear limitation, once the speed limiter system has been deactivated, downward transmission gear changes are prevented or at least limited, so that engine torque and vehicle speed rise smoothly to the value corresponding to the actual accelerator pedal position. Steep rises or sudden changes in the engine torque are avoided, so that driving safety is objectively and subjectively improved.
In an advantageous embodiment, the time period is measured within which a minimum gear is preset as the low gear limit, based on the ratio of the actual accelerator pedal position to the accelerator pedal substitute variable. The low gear limit is maintained, after the cruise control has been switched off, for as long as the actual accelerator pedal position is greater than the accelerator pedal substitute value or some other variable which correlates with the vehicle speed or the engine torque. In this situation, the driver would like a torque that is greater than the actual engine torque; the presetting of the minimum gear is not cancelled until the torque desired by the driver has been reached.
Both the actual accelerator pedal position and the accelerator pedal substitute variable each have an engine torque associated with them internally in the speed limiter system or in some other vehicle regulation and control unit, and the rise in the accelerator pedal substitute variable is carried out in accordance with a preset engine torque characteristic until the actual accelerator pedal position is reached. Presetting the torque profile allows the vehicle behavior to be influenced optimally.
The minimum gear is determined as a function of vehicle state variables, in particular as a function of the preset vehicle maximum speed. Within the time period under consideration, in which the actual accelerator pedal position is greater than the accelerator pedal substitute variable, the minimum gear can be defined as a function of state variables as a predetermined functional profile. In particular, a staircase function is used for the minimum gear as a function of the selected maximum speed.
The actual vehicle operating modexe2x80x94in particular normal operation or kick-down operationxe2x80x94can also be taken into account. In kick-down operation, account is taken of the driver""s wish for a faster increase in speed; for this purpose, a different profile is used for the minimum gear, in which a next-higher minimum gear is advantageously set for preset maximum speeds which are greater than in normal operation, so that increased engine speeds are reached during acceleration.
Immediately after the cruise control has been switched off, the actual gear which is selected in the transmission at the time when the system is switched off is initially maintained for a given time period. This forces a dead-time response, in which a gear change is carried out only after a time delay. However, it may be expedient to provide for this dead-time response to be overridden by specific driver reactions, for example kick-down operation.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.